The present invention relates to a method for making keto acids by oxidatively cleaving an .alpha.,.beta.-unsaturated ketones such as isophorone which thereafter can be converted to dimedone by the dehydration of the resulting keto acid.
Prior to the present invention, dimedone, or 5,5-dimethylcyclohexane-1,3-dione, as shown by the following formula, ##STR1## was used to make various silyl ethers, in accordance with my invention, as shown by U.S. Pat. No. 4,210,596. These silylethers are useful as RTV curing agents for making moisture curable organopolysiloxanes, as shown by U.S. Pat. Nos. 4,176,111 and 4,176,112, assigned to the same assignee as the present invention. One method for making dimedone is based on the cyclization of the corresponding keto acid or ester, as shown by Blaha et al, Czech. Patent 116,788 and Blaha et al, Collection Czech. Chem. Comm., 30, 1214 (1965). The aforementioned procedure of Blaha, however, is economically unattractive due to the high material costs, since the keto acids which are cyclized generally have been obtained by a condensation reaction of an acetic acid equivalent and mesityl oxide, or an acrylate with an acetone equivalent. Considerable effort has therefore been expended to determine satisfactory methods for making 5-keto-3,3-dimethylhexanoic acid which is the precursor of dimedone.
The present invention is based on the discovery that isophorone, an .alpha.,.beta.-unsaturated ketone having the formula, ##STR2## can be oxidatively cleaved with loss of the alpha carbon atom to produce 5-keto-3,3-dimethylhexanoic acid. In view of the fact that isophorone is a readily available commercial material derived from the condensation of acetone, a satisfactory technique for synthesizing dimedone is therefore available.
I have further discovered that a particularly attractive procedure for oxidatively cleaving isophorone and other .alpha.,.beta.-unsaturated ketones is by the use of ozone. I have further found that cleavage of the resulting ozonide formed during the ozonolysis of the isophorone can be satisfactorily achieved by the use of hydrogen peroxide and an alkali metal hydroxide and a phase transfer catalyst, whereby the resulting keto acid is selectively extracted from the organic phase to the aqueous phase as it forms. As a result, there is no build-up of a potentially dangerous ozonide since it is readily decomposed as it is formed. The resulting keto acid can then be isolated satisfactorily by acidification of the aqueous phase.